1. Field of the Invention
The present invention relates to a pressure regulator for regulating pressure in a pressure chamber used for pressing a substrate, such as a wafer, against a polishing pad. The present invention further relates to a polishing apparatus having such a pressure regulator. The present invention further relates to a polishing method using the aforementioned polishing apparatus.
2. Description of the Related Art
FIG. 1 is a schematic view of a polishing apparatus for polishing a wafer. As shown in FIG. 1, the polishing apparatus has a polishing table 22 for supporting a polishing pad 23, and a top ring 30 for pressing a wafer W against the polishing pad 23. The polishing table 22 is coupled to a table motor 29, which is provided below the polishing table 22, through a table shaft 22a. This table motor 29 is configured to rotate the polishing table 22 in a direction indicated by arrow. The polishing pad 23 is attached to an upper surface of the polishing table 22, and an upper surface of the polishing pad 23 serves as a polishing surface 23a for polishing the wafer W. The top ring 30 is secured to a lower end of a top ring shaft 27. The top ring 30 is configured to hold the wafer W on its lower surface via vacuum suction.
Polishing of the wafer W is performed as follows. The top ring 30 and the polishing table 22 are rotated in directions as indicated by arrows, while a polishing liquid (i.e., slurry) is supplied onto the polishing pad 23 from a polishing-liquid supply unit 25. In this state, the top ring 30, holding the wafer W on its lower surface, is lowered and presses the wafer W against the polishing surface 23a of the polishing pad 23. A surface of the wafer W is polished by a mechanical action of abrasive grains contained in the polishing liquid and a chemical action of the polishing liquid. Such polishing apparatus is known as CMP (chemical mechanical polishing) apparatus.
The top ring 30 has its lower portion constituted by a pressure chamber (not shown in FIG. 1) which is formed by a flexible membrane. A pressurized gas is supplied into the pressure chamber so that polishing pressure on the wafer W against the polishing pad 23 is regulated by the pressure in the pressure chamber. FIG. 2 is a schematic view showing a pressure regulator 100 for regulating the pressure in the pressure chamber by supplying a gas (e.g., air or nitrogen gas) into the pressure chamber of the top ring 30. As shown in FIG. 2, the pressure regulator 100 has a pressure-regulating valve 101 for regulating the pressure of the gas supplied from a gas supply source, a pressure sensor 102 for measuring the pressure (i.e., the secondary pressure) of the gas downstream of the pressure-regulating valve 101, and a regulator controller 103 for controlling operation of the pressure-regulating valve 101 based on a pressure value obtained by the pressure sensor 102. The pressure regulator 100 having such structures is known as an electropneumatic regulator.
The pressure-regulating valve 101 has a pilot valve 110 for regulating the pressure of the gas supplied from the gas supply source, and a gas-intake electromagnetic valve 111 and a gas-release electromagnetic valve 112 each for regulating pressure of a pilot air to be supplied to the pilot valve 110. The pilot valve 110 has a pilot chamber 115 and a valve element 116 coupled to the pilot chamber 115. A part of the pilot chamber 115 is formed from a diaphragm. The pilot air is supplied into the pilot chamber 115 through the gas-intake electromagnetic valve 111 and is discharged from the pilot chamber 115 through the gas-release electromagnetic valve 112. Therefore, the pressure in the pilot chamber 115 is controlled by operating the gas-intake electromagnetic valve 111 and the gas-release electromagnetic valve 112. The regulator controller 103 controls open-close operations of the electromagnetic valves 111, 112, and the valve element 116 is moved according to the pressure in the pilot chamber 115. Depending on the position of the valve element 116, the gas from the gas supply source passes through the pilot valve 110 or the gas downstream of the pilot valve 110 (i.e., the gas on the secondary side) is discharged through the pilot valve 110, so that the pressure of the gas existing downstream of the pilot valve 110 (i.e., the secondary pressure) is regulated.
The regulator controller 103 is coupled to a polishing controller 50 of the polishing apparatus, and is configured to receive a pressure command value which is sent from the polishing controller 50. The regulator controller 103 controls the operations of the gas-intake electromagnetic valve 111 and the gas-release electromagnetic valve 112 so as to eliminate a difference between a pressure current value of the gas measured by the pressure sensor 102 and the pressure command value to thereby adjust the pressure in the pressure chamber of the top ring 30.
However, when the pressure sensor 102 is affected by disturbance (e.g., temperature change), an output value of the pressure sensor 102 may deviate from an actual pressure. Such a deviation of the output value is called a temperature drift. Other than the temperature drift, a sliding resistance of the pilot valve 110, a measuring accuracy of the pressure sensor 102 itself, and a distance between the pressure regulator (i.e., electropneumatic regulator) 100 and a point of use may cause an error in the output value of the pressure sensor 102. Since the regulator controller 103 is operated such that the output value of the pressure sensor 102 is kept at the pressure command value, the actual gas pressure adjusted by the pressure regulator 101 may differ from the pressure command value. Moreover, if the polishing pressure on the wafer is controlled based on the pressure that differs from the actual pressure, an intended polishing result may not be obtained.